MRI scanners, which are used in various fields such as medical diagnostics, typically use a computer to create images based on the operation of a magnet, a gradient coil assembly, and a radiofrequency coil(s). The magnet creates a uniform main magnetic field, which makes nuclei, such as that of hydrogen atoms, responsive to radiofrequency excitation. The gradient coil assembly imposes a series of pulsed, spatially varying magnetic fields upon the main magnetic field, in order to give each point in the imaging volume a spatial identity corresponding to its unique magnetic field values during the imaging pulse sequence. The radiofrequency coil(s) generate(s) an excitation frequency pulse that causes a temporary oscillating transverse magnetization of the nuclei. The nuclei relaxation from that states results in an emitted signal which is detected by the radiofrequency coil and used by the computer to create the image.
The typical MRI system is provided with shielding means designed to prevent exposure to static stray magnetic field to the operator, surrounding equipment and facilities. The typical stray magnetic field limit imposed by the U.S. Food and Drug Administration (FDA) with respect to external personnel exposure is 5 gauss; additional stray field limitations of higher values may be imposed in the design to prevent interference with electronics and other nearby equipment.
The shielding means of superconductive magnets may include superconductive bucking coils (active shielding) and/or ferromagnetic (iron) shielding elements. The superconductive bucking coils carry electric currents of generally opposite direction to the electric current carried in the superconductive main coils. The superconductive bucking coils are positioned radially outward from the superconductive main coils to counterbalance magnetic moments created by the main coils. Likewise, the cylindrical iron shield is positioned radially outward from the superconductive main coils to prevent leakage outside the magnet of the magnetic field created by the main coils.
Orthopedics is a medical specialty concerned with correction of deformities or functional impairments of the skeletal system, particularly, of the extremities and the spine, and associated structures, such as muscles and ligaments. For example, diagnosis and treatment of broken hand or leg bones is a common practice in orthopedics. Because many orthopedic health problems are subcutaneous, imaging anatomy under the skin is a very important capability in orthopedics. Magnetic resonance imaging (MRI) is one imaging technique implemented in orthopedic diagnosis.
Use of conventional whole body (WB) MRI systems in orthopedic imaging has its intrinsic limitations. The distance between the front face and the field of view is hardly sufficient to allow the patient to extend their arm or leg into the centrally located field of view from the outside of the MRI system, therefore patients must egress into the center of the MRI even for orthopedic imaging of limbs. For claustrophobic patients, this can be a traumatic experience. In addition, the large size and large stray field footprint of conventional WB MRI systems require a large floor space in which to site the MRI system and associated increased facilities cost. Furthermore, conventional full body MRI systems have higher cost than the dedicated orthopedic system, as the large bore inevitably leads to much larger overall dimensions, forces and amount of superconductor, as well as complexity of structural and cryogenic designs.
Thus, in the case of orthopedic application for MRI systems there are considerable advantages in terms over cost and sitability for small diameter bore magnet systems dedicated to extremity imaging such as human legs and arms. However, an orthopedic MRI system suffers from the limitation that the shielding increases the outside dimensions of the magnet system which limits the access for leg imaging due to the difficulty of positioning the subject's second leg outside the imaging region.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for orthopedic imaging that is not limited by the outside dimensions of the magnet system. There is also a need for improved magnetic resonance imaging of extremities, which does not compromise or affect the accuracy or operation of the MRI.